Controls on the Sedimentological Heterogeneity of Visean Carbonate Platforms in England and Wales

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Carbonate rocks (limestones) can record the depositional environments of prehistoric seas. Theposition of the sea in space and time controls the types of organisms which live within it, how they aredistributed, and how they build into a body of sediment with topographic relief, known as a carbonateplatform. This is because limestones are built from the calcium-carbonate skeletons of marine organismswhich are highly sensitive to light, nutrition, temperature, and salinity (the "carbonate factory"). Duringthe Lower Carboniferous Period (362.5 million years ago), tectonic plates reassembled by continental driftto build supercontinent Pangaea. This led the UK to be positioned at around 14 degrees south of the equator,comparable to the present-day position of Brazil or Madagascar. The north of the UK was positionedunder the sea, giving the potential for limestones to be deposited.The carbonate platforms were deposited in two stages over approximately 11.1 million years: theAsbian (oldest) and the Brigantian (youngest). Asbian rocks (strata) were laid down during a greenhouseclimate, defined as a world with no ice at the poles. This meant that sea-level did not fluctuate muchbecause sea water was not periodically frozen into ice caps. A large ice mass (the Gondwanan ice sheet)began to develop at the south pole during Asbian, and an Icehouse climate had established by theBrigantian. As a result, sea-level fluctuated considerably by the Brigantian, in terms of frequency andamplitude, and seasons (e.g. periodic wet/dry seasons) were amplified. This meant that the carbonatefactory existed under more stressed conditions and was influenced by systems on land (terrestrial), suchas rivers and deltas.As well as a significant change in climate, the carbonate platforms were also subjected to activetectonic movement. During the Asbian, the rocks were being pulled apart, to form a series of faultboundedblocks (half-grabens) which are 1000s of km2 in size, which have topographic highs. The bestconditions for organisms to thrive was upon the highest topography, where light and temperature wereoptimal. By the end-Brigantian, the strata had been pulled apart enough for thermal subsidence to occur,whereby the cooling mantle (beneath the crust) contracted to become part of the lithosphere (crust andupper mantle) which is denser than the surrounding material and consequently subsided and sea-levelincreased around the carbonate platform.